This invention refers to a centralized device for the so called ventilation, namely, the distribution and discharge of air intended to cause the evaporation of a carrier in user installations, such as sections of rotogravure machines, transformation machines and the like.
This invention will be described with particular reference to rotogravure machines, which represent a typical application thereof, however this specific example of application has not to be regarded as a limitation for a more general application of the invention.
In the rotogravure machines, the different colors of a subject are printed sequentially on a support, by operating in subsequent sections of the machine, whose number may vary, for example, from three to ten and more. The ink or lacquer used for printing is diluted with a carrier, in most cases a solvent, whose purpose is that of conferring to the ink or lacquer a suitable viscosity and allowing to obtain the desired color tonality in the print. After print of each color, and before initiating the print of the subsequent color, this carrier should be removed by evaporation. In order to perform this operation, the printing machine comprises a number of evaporation tunnels, each tunnel being interposed between a pair of subsequent print sections. In each evaporation tunnel, an air stream is blown by means of special nozzles on the just printed face of the support, in order to cause the evaporation of the carrier, and the air-vapor mixture thus formed is discharged from the tunnel.
The delivery rate, velocity and temperature of the blown air should be suitably chosen in each tunnel, on one hand in order to ensure the complete evaporation of the carrier during the permanency period of the support in the tunnel, and on the other hand in order to obtain a suitable concentration of the vapor in the discharged air-vapor mixture.
In most cases, the carrier to be evaporated is a solvent forming with the air an inflammable mixture, which in certain cases is detonating too. It is therefore needed, for the sake of security, that the discharged mixture contains a vapor concentration lesser, according to a suitable security coefficient, than the lower detonation limit of the mixture. On the other hand, the regulations concerning the ambient protection against pollution do not permit to simply discharge in the atmosphere the mixtures of air and solvent discharged from the tunnel, and therefore these mixtures should be directed to purifying apparatuses for recuperating the solvent, or to removal apparatuses for its destruction, which usually is obtained by combustion. The management burden of these purifying apparatuses increases with the quantity of treated mixture, and is the more high the less is the concentration of the solvent in the mixture. As a consequence, it is important that, though respecting the security requirements, the concentration of solvent in the treated mixture has the higher possible value.
In order to satisfy these requirements, each tunnel is provided with its own air distribution circuit which is regulated, manually or by means of automatic devices, in such a way as to introduce, discharge and recycle suitable air quantities, selected as functions of the quality and quantity of solvent which has to be evaporated in the considered tunnel. The carrier quantities to be evaporated in the subsequent tunnels of the same rotogravure machine during the print of the same subject may be very different, due to the fact that the surfaces covered by the different colors of the printed subject may have extensions very different and, therefore, may give rise to quantities correspondingly different of carrier to be evaporated in the various sections. Therefore, also in view of the requirements of the various sections of the machine, which may be very different, the optimal conditions cannot be attained by a sole distribution circuit which operates all the tunnels without introducing differences among them.
As already said, the specific requirements stated for the rotogravure machines, or similar requirements, are encountered also in other installations, such as the transformation machines and others.
Presently, the rotogravure machines often use a device for air distribution, recirculation and discharge with respect to the printing elements, which is provided with an air supply channel, a fan, a discharge channel and, if needed, a heat exchanger, and this device is centralized for a number of elements (or, more generally, for a number of user installations). This device comprises a delivery manifold connected to the air supply channels of the various installations, a mixture suction manifold, connected to the discharge channels of the various installations, a discharge fan inserted in this mixture suction manifold, a channel for direction to the discharge, a fresh air suction channel, a valve system controlled for the regulation of the air streams, and a continual analyzer connected to the mixture suction manifold and intended to control the valve motors.
Thanks to these features the device, through its delivery manifold, supplies partially recycled air to a number of installations, and each installation uses the same by means of its own fan which is regulated according to the specific requirements of that installation, and if needed this air is heated by means of the own heat exchanger of the installation; then the air-solvent mixture discharged by the various installations is collected by the mixture suction manifold and mixed with the mixtures discharged by the other connected installations. The resulting mixture is analyzed in order to measure the carrier concentration, and three valves, present in the system, are operated by their motors under control of the analyzer in such a way as to direct to the discharge, by means of the discharge fan, a certain fraction of this mixture, to recycle another fraction of the mixture, by directing the same to the delivery manifold, and to add to the recycled mixture fraction a certain quantity of fresh air coming from the fresh air suction channel.
However, this system suffers certain limits. Particularly, its operation is scarcely effective in those cases in which two or more elements of the installation are charged to evaporate quantities of solvent very larger than the mean quantity evaporated in the other elements. In this case, the low values of recirculation allowable for that elements limit the recirculation directed to other elements which, on the contrary, in view of a low concentration of solvent at their discharge, could operate with a higher ratio of recirculation, thus taking advantage of the thermal recuperation and directing a lower air quantity to the discharge. These limits impose the use of an air distribution, recirculation and discharge system independent from the centralized system, for those elements in which prints or spreads are foreseen, which involve high quantities of solvent.